Quantitative Determination of the Regioselectivity of Nucleophilic Addition to Î·3-Propargyl Rhenium Complexes and Direct Observation of an Equilibrium between Î·3-Propargyl Rhenium Complexes and Rhenacyclobutenes.

Casey, Charles P.

Boller, Timothy M.

Samec, Joseph S. M.

Reinert-Nash, John R.

Abstract [en]

PMe3 adds selectively to the central C of the Î·3-propargyl complex [C5Me5(CO)2Re(Î·3-CH2Câ‰¡CCMe3)][BF4] (1-t-Bu) to form the metallacyclobutene [C5Me5(CO)2Re(CH2C(PMe3):CCMe3)][BF4] (7). The rate of rearrangement of the metallacyclobutene 7 to Î·2-alkyne complex [C5Me5(CO)2Re(Î·2-Me3PCH2Câ‰¡CCMe3)][BF4] (8) is independent of phosphine concn., consistent with a dissociative mechanism proceeding via Î·3-propargyl complex 1-t-Bu. The rate of this rearrangement is 480 times slower than the rate of exchange of PMe3 with the labeled metallacyclobutene 7-d9. This rate ratio provides an indirect measurement of the regioselectivity for addn. of PMe3 to the central C of Î·3-propargyl complex 1-t-Bu to give 7 compared to addn. to a terminal C to give 8. The addn. of PPh3 to 1-t-Bu gives the metallacyclobutene [C5Me5(CO)2Re(CH2C(PPh3):CCMe3)][BF4] (11). Low-temp. 1H NMR spectra provide evidence for an equil. between metallacyclobutene 11 and Î·3-propargyl complex 1-t-Bu (Keq â‰ˆ 44 M-1 at -46Â° and Î”GÂ°(0Â°) = -1.2 Â± 0.2 kcal mol-1). The crystal and mol. structures of [C5Me5(CO)2Re[Î·2-(Ar2PCH2CH2PPh2)CH2Câ‰¡CCMe3]][BF4]Â·2CH2Cl2 (Ar = 3,5-(CF3)2C6H3) and [C5Me5(CO)2Re(Î·2-Me3PCH2Câ‰¡CCMe3)]ClÂ·H2O were detd. by x-ray crystallog. [on SciFinder(R)]